The behavior of many systems in technology is investigated by means of demodulation techniques, whereby the system is excited with a modulated signal and the properties of the reflected radiation are used to determine the characteristics of the system under investigation.
A particular example of measuring techniques which require the demodulation of a temporally modulated electromagnetic radiation field are the time of flight (TOF) distance measurements, based on the fact that the speed of light is finite and constant in a given medium. In a time of flight experiment, the scene is illuminated by a high frequency modulated light source which is usually located near the detector. The signal reflected from the object travels back to the detector with a delay proportional to the distance. After the demodulation of the received signal, the phase difference between the received and sent signal is obtained, and the distance to the object can be calculated. The distance of the object can be expressed asD=c·TOF/2
Several sensor architectures with demodulation capabilities were proposed so far. A CCD based device capable of demodulation of the impinging light signal was described in Spirig, “Apparatus and method for detection and demodulation of an intensity-modulated radiation field”, U.S. Pat. No. 5,856,667. Another example of the similar approach is described in Seitz, “Four-tap demodulation pixel” GB 2389960. The light impinging on the device creates hole-electron pairs in the bulk of the semiconductor, those that diffused to the top of the sensor are forwarded to the destination potential storage well by means of the lateral electric field under the gates. The drawbacks of this approach are the slow diffusion of the photo-generated charge carriers to the top of the sensor, the presence of the gates over the sensitive area which reduce the responsivity of the sensor and the large gate capacitance that should be driven on each toggling of the control voltage. The same slow diffusion process to the top of the sensor is present in the demodulation device described in Buettgen, “device and method for the demodulation of modulated electric signals”, US20080247033, where the photo-generated charge carriers are forced to drift to a particular detection region by providing an alternating potential to different gate contacts of a demodulation gate structure.
Another approach of the implementation of the demodulating sensor is disclosed in Kuijk, “Detector for electromagnetic radiation assisted by majority current”, U.S. Pat. No. 6,987,268. The electric field induced in the substrate lifts up the deeply generated carriers and accelerates them in the direction of the detection nodes. The drawback of this approach is the alternating current which has to be supplied to maintain an alternating electric field in the substrate.